I've been where you are at.
For your freelance + job question...If you know yourself as someone that is good with time management, very resourceful, and very effective.. then maybe it could work doing both @ 100% quality.
But think seriously about your life now, what's important to you. Doing this will likely absolutely consume your life. Is there a girlfriend depending on you to spend time with them? If so, be prepared to upset her and create lots of problems in the relationship. Of course that depends on the relationship.
Now, really... to know whether you are READY to do a startup.. the best way is to work on your plan and create some proof of concept for yourself (and for investors if you need them). Stay @ your job if its not too much suffering until you can minimize the risk of failure to the point that you can say.. "YES, there is a high chance that customers will buy what I am selling at this price.. I have proof right here". That is the best time to quit your job and go for it.. because you are making a calculated bet.
Now, if you don't need to support yourself, or support another.. ie. someone else can pay for your bills and feed you, and give you some money to socialize.. then there is little downside to quitting and doing your own thing. Just make sure you quit respectfully and honestly.. most employers will respect that and admire you for it.
I spoke to a top VC in san fran one time, and he said the fastest way to accelerate towards the goal of being a successful entrepreneur is to get down and dirty. Not by being in a company doing a very focused specific job.
One thing... don't let the whole economic downturn bother you.. There is so much opportunity out there.. this might even be one of the best times to start a business.
Also, if you wait too long.. you will tire out and maybe even drown out your dream. If you need to itch your bite, then itch it as soon as possible.
Good luck.
If you want to chat.. let me know.